The final piece to the puzzle was provided by Josh Emery of the University of Tennessee, Knoxville, who used NASA's Spitzer Space Telescope in 2007 to study the space rock's thermal characteristics. Emery's measurements of the infrared emissions from 1999 RQ36 allowed him to derive the object's temperatures. From there he was able to determine the degree to which the asteroid is covered by an insulating blanket of fine material, which is a key factor for the Yarkovsky effect.

With the asteroid's orbit, size, thermal properties and propulsive force (Yarkovsky effect) understood, Chesley was able to perform the space rock scientist equivalent of solving for "X" and calculate its bulk density.

"While 1999 RQ36 weighs in at about 60 million metric tons, it is about a half kilometer across," said Chesley. "That means it has about the same density as water, so it's more than likely a very porous jumble of rocks and dust."

Asteroid 1999 RQ36 is of particular interest to NASA as it is the target of the agency's OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) mission. Scheduled for launch in 2016, ORIRIS-Rex will visit 1999 RQ36, collect samples from the asteroid and return them to Earth.

NASA detects, tracks and characterizes asteroids and comets passing relatively close to Earth using both ground- and space-based telescopes. The Near-Earth Object Observations Program, commonly called "Spaceguard," discovers these objects, characterizes a subset of them, and establishes their orbits to determine if any could be potentially hazardous to our planet. JPL manages the Near-Earth Object Program Office for NASA's Science Mission Directorate in Washington. JPL is a division of the California Institute of Technology in Pasadena. JPL also manages the Spitzer Space Telescope and Goldstone Solar System Radar.